The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The present disclosure relates to a transmission within a vehicular powertrain having a selectable one-way clutch used to selectively transfer torque from an input member to an output member. One potential embodiment includes a selectable one-way clutch, utilized in combination with rotating input clutches that are slipped for engagement in forward and reverse. Clutch engagements can be performed in by a number of known methods. In a friction clutch engagement configuration, one or more clutches inside the transmission are slipped and gradually engaged to gradually transfer torque from the engine into the transmission for launch. In a frictionless clutch engagement configuration, clutch engagement occurs without clutch slip, requiring that the clutch be synchronized before engagement occurs.
A transmission operates in one of a plurality of transmission gear states, defining the direction of torque applied to the output member and a gear ratio describing the relationship of torque applied to the input member to the output member. An exemplary transmission includes eight transmission gear states comprising six forward gears, a reverse gear, and a neutral gear state.
A vehicle powertrain equipped with an automatic transmission can include a torque converter positioned between the engine and the transmission. In the alternative, a flywheel and isolator configuration can be used between the engine and the transmission. Some automatic transmissions include a low/reverse braking clutch and a one way clutch to provide reaction torque in both forward and reverse directions as input clutches are slipped for launching the vehicle without a torque converter.
One-way clutches are used in numerous applications to selectively transmit power from a first rotatable driving member or input member such as an input race or first coupling plate, to a second independent driven member or output member such as an output race or second coupling plate. In application, the clutch transmits torque when the rotation of the driving member with respect to the driven member is in a first direction. When the relative rotational direction of the driving member with respect to the driven member is in an opposite or second direction, the clutch will release or decouple the driving member from the driven member. Once released, the driving member is free to rotate in the second opposite direction relative to the driven member. In this mode, the one-way clutch is free to overrun or “freewheel”. Freewheeling enables the vehicle to travel in a coasting movement, with the drivetrain decoupled from the engine, thereby eliminating losses associated with the drivetrain turning the engine.
Engine braking is an operation whereby torque to the driving member is reduced, a coupled connection between the driving member and the driven member is maintained, and back-torque as a result of turning an engine with reduced power applied and pumping forces in the engine is transmitted through the connection with a result of slowing the driven member. Freewheeling of a one-way clutch decouples the driving member and the driven member inconsistent with operation of engine braking. Engine braking in a vehicle equipped with a one-way clutch operating as described above requires additional components, for example, an additional clutch to selectively couple the driving member and the driven member, to accomplish engine braking.
In an exemplary front wheel drive, six speed automatic vehicle transmission, using a three gear set and five torque transmitting element (clutch) arrangement, one of the clutches, henceforth referred to as the low and reverse clutch, is only applied in manual low and in reverse. In first gear in the drive mode, the low and reverse clutch is disengaged. There is no relative motion between the input and output components of the low and reverse clutch because a reaction torque acting on a one-way clutch, to which the input components of the low and reverse clutch are connected, prevents rotation. However, in all other forward gears, second through sixth in the drive mode, there is no reaction torque acting on the one-way clutch as a result of one or more other clutches in the transmission being applied. Consequently, the one-way clutch freewheels and relative motion between the low and reverse clutch components is present. The relative speed between the input and output components of the low and reverse clutch increases with each successive gear change.
As is commonly known to those familiar with the art, disengaged multi-plate clutches, depending on various conditions, produce drag when there is relative motion between the input and the output members. One characteristic of a disengaged multi-plate clutch is that, as the relative speed between the input and output members increases, typically, so does the drag or spin loss. This spin loss contributes to decreased fuel economy. Since the low and reverse clutch is disengaged in all of the forward gears of the described exemplary six speed automatic transmission with this particular gear set and clutch arrangement, and since most of the transmission's operating mode is in the forward gear ratios, a measurable amount of the overall spin losses in the transmission can be attributed to the drag in the disengaged low and reverse clutch.
A selectable one-way clutch (SOWC) is similar to a one-way clutch in basic operation. However, depending on the details of the design, an SOWC is capable of producing a mechanical connection between a driving and a driven member in one or both rotational directions. Also, depending on the design, an SOWC is capable of overrunning in one or both directions of rotation. Typically, a SOWC contains an externally controllable selection mechanism, that when moved to a second, and possibly even a third position, controls the operating modes of the device. An SOWC, replacing a one-way clutch and a low and reverse clutch configuration as described above, reduces the part complexity of the transmission, reduces weight and cost of the transmission, and reduces overall spin losses.
The ability of the driving member of a SOWC to transmit torque to a driven member depends on whether or not the engaging torque transmitting elements of the SOWC, typically rollers, sprags, rockers, or struts, etc., are able to freely engage one or more cams, notches, recesses, or similar features in the adjacent member. Similarly, the ability of the driving member to freewheel in one or both directions with respect to the driven member also depends on whether or not the engaging elements are free to interact with the adjacent member. Typically, a rotatable retainer ring or selector plate is used to accomplish this task by allowing or restricting engagement of the torque transmitting elements between the driving and the driven member. Other similar clutch designs are known in the art and capable of use as one-way clutching devices, including synchronizers and dog clutches. A number of clutch designs capable of functioning as an SOWC are envisioned, and this disclosure is not intended to be limited to the particular exemplary embodiments described herein.
Depending on the application, an SOWC can be designed to operate in two modes or three modes. This disclosure will focus on a two mode SOWC. In particular, this disclosure will reference an SOWC enabling a first, disengaged mode and a second, engaged mode. The disengaged mode includes operation of the SOWC as a one-way clutch, as described above, allowing transmission of torque in a forward direction and freewheeling in the reverse direction. The engaged mode includes locking the clutch, preventing relative rotation of the driving member in either direction. In the engaged mode, the clutch allows transmission of torque in forward and reverse directions. Selection between the two modes includes actuation of a selection mechanism, for example, rotating a retaining ring or selector plate, henceforth simply referred to as a selector plate, between annular positions.
As described above, transitioning a frictionless engagement SOWC from a disengaged state to an engaged state requires a condition of zero slip in the SOWC. Zero slip requires that the driving member and the driven member are at the same speed. The driven member is directly connected to the drivetrain including the wheels of the vehicle. The speed of the driven member cannot be abruptly changed without affecting vehicle movement and drivability. A method to control the speed of the driving member to achieve zero slip in an SOWC would be beneficial.